In recent years, portable devices driven by secondary batteries such as a lithium ion battery are popularly used along with a tendency of compactness of electronic devices. In order to avoid non-availability of such portable devices, a remaining battery level may be indicated on a display or the like. A user of the portable device uses the portable device while considering the remaining battery level with the indication and can charge the secondary battery at an appropriate timing. For example, Patent Document 1 discloses a secondary battery unit having a LDC display part on which a measured remaining battery level of a battery is indicated and a remaining battery level measuring method for a secondary battery.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2007-309943